1. Field of the Invention
The present invention is a Continuation of application Ser. No. 15/198,832, filed Jun. 30, 2016, the entire contents of which are incorporated herein by reference.
This application relates generally to sample-and-hold circuits. More specifically, this application relates to a sample-and-hold circuit that can reduce the effects of RTS noise in image sensing or other electronic applications.
2. Description of Related Art
Image sensing devices typically consist of an image sensor, generally an array of pixel circuits, as well as signal processing circuitry and any associated control or timing circuitry. Within the image sensor itself, charge is collected in a photoelectric conversion device of the pixel circuit as a result of the impingement of light.
One example of a pixel circuit is illustrated in FIG. 1. As shown in FIG. 1, a pixel circuit 100 includes a photoelectric conversion device 101 (for example, a photodiode), a floating diffusion FD, a transfer transistor 102, a reset transistor 103, an amplification transistor 104, and a selection transistor 105, and a vertical signal line 106. As illustrated, vertical signal line 106 is common to a plurality of pixel circuits within the same column. Alternatively, a vertical signal line may be shared among multiple columns. Gate electrodes of transfer transistor 102, reset transistor 103, and selection transistor 105 receive signals TRG, RST, and SEL, respectively. These signals may, for example, be provided by the control or timing circuitry.
While FIG. 1 illustrates a pixel circuit having four transistors in a particular configuration, the current disclosure is not so limited and may apply to a pixel circuit having fewer or more transistors as well as other elements, such as capacitors, resistors, and the like. Additionally, the current disclosure may be extended to configurations where one or more transistors are shared among multiple photoelectric conversion devices.
The accumulated charge is then converted to a digital value. Such a conversion typically requires several circuit components such as sample-and-hold (S/H) circuits, analog-to-digital converters (ADC), and timing and control circuits, with each circuit component serving a purpose in the conversion. For example, the purpose of the S/H circuit may be to sample the analog signals from different time phases of the photo diode operation, after which the analog signals may be converted to digital form by the ADC.
However, in high-resolution image sensors, it is desirable to keep the noise source in the various circuit components to a minimum. A significant noise source in typical sample-and-hold circuit implementations is the 1/f noise in a MOS transistor; for example, an NMOS transistor. Such noise originates from the oxide-trap-induced charge mobility fluctuations of the transistor. For MOS transistors with a small channel area, such as those used in some sample-and-hold circuits, the capture and emission of a carrier by the trap results in a current that resembles a random telegraph signal (“RTS”). The RTS noise in the S/H transistor appears as pixel noise in the resulting images and thus has a significant impact on the image quality of the sensor. Therefore, there exists a need for a method of signal processing that can reduce the impact of RTS noise.